ABSTRACT The protozoan parasite Entamoeba histolytica causes an estimated 50 million cases of invasive disease annually and is the second leading parasitic cause of death worldwide. The most common manifestations of amebic infection are colonic disease (dysentery) and liver abscesses. In addition to the lives lost there is significant morbidity associated with amebic diarrheal illness, especially in children who often suffer from malnourishment and growth retardation. E. histolytica is classified as a Class B agent of concern for bioterrorism. The parasite?s life cycle involves interconversion between the trophozoite and cyst stages. This interconversion, which is essential for disease propagation and pathogenesis, is extremely poorly understood. Despite efforts over decades by numerous groups, high-grade encystation has not been developed for the lab strains of E. histolytica. Instead, the reptilian parasite E. invadens has been used as an important model to study the developmental cascade in Entamoeba. These efforts in E. invadens have defined important regulatory features and have moved the system forward with important mechanistic insights. However, the work is still one step away from giving insights in the human parasite E. histolytica. We now have an opportunity to define features of E. histolytica cysts and to take an important step forward in developing an in vitro model for E. histolytica encystation. The Knoll lab has identified a mouse model whereas the genome sequenced lab strain of E. histolytica (strain HM-1:IMSS) can be gavage fed into mice to generate cysts in the stool. This is the first time that cysts of E. histolytica have been generated from a lab-adapted genome strain that is able to be genetically manipulated. Our goals are to characterize the mouse- derived cysts and to use them to develop tools to study E. histolytica developmental biology. To further these goals our plans are: Aim 1: Characterize cysts from the mouse model and develop a genome-wide transcriptional profile of E. histolytica cysts and Aim 2: use the mouse-derived cysts to develop trophozoite cultures that are capable of regulated in vitro encystation. In the first Aim, we will characterize the cysts by staining with antibodies and nuclear stain and perform RNASeq transcriptional profiling. These efforts will provide an important dataset and will be useful to identify markers for E. histolytica cysts and to define the mature cyst composition. In the second Aim, we will determine whether parasites that have been encysted in an animal model can be adapted to trophozoite culture and used to develop in vitro encystation. Successful development of in vitro encystation will allow us to dissect key aspects of the developmental biology of this parasite including identification of the pathways that trigger encystation. These approaches promise to provide crucial insights into differentiation in E. histolytica and will pave the way for further characterization of the development program in this parasite.